(a) Field of the Invention
This invention relates to a sound field control apparatus producing a desired sound field in reproduction of two channel stereophonic input signals.
(b) Description of the Prior Art
In the past, a sound field control apparatus of this type has been used in general with the intention of reproducing a sound field which is accommodated in a sound source in recording, as exactly as possible. Alternatively, however, an apparatus producing desired sound fields such as music halls and churches, independently of the sound field in recording, has recently been developed (Refer to, Radio Technology, "Creating sound field--the world of omnisound with YAMAHA DSP-1", Aug. 20, 1986).
Such an apparatus is adapted to store time delay and level of a signal, as sound field data, to be supplied to each of loudspeakers for sound field reproduction in order to reproduce imaginary sound source distribution previously measured in, for example, famous music halls in the world, execute convolution operation processing with the sound field data for two channel stereophonic input signals derived from the sound source in reproduction, and output an initial reflection sound signal group produced for each loudspeaker to a plurality of sound field reproducing loudspeakers.
To make sound field creation with this system, such a fundamental arrangement as shown in FIG. 1 is suggested. In this figure, signals L, R for individual channels of two channel stereophonic signals inputted to input terminals lL, lR are supplied to loudspeakers LS, RS for a sound source which are placed on the left and right side, respectively, through power amplifiers not shown, and are converted into sound thereat. On the other hand, a digital sound processor (DSP) 2 convolutes the sound field data stored in a memory not shown to the sum or difference (a difference signal (L-R)is shown in FIG. 1) of the individual channel signals and outputs such an initial reflection sound signal group shown in FIG. 2 to loudspeakers FL, FR, RL, RR for exclusive use of sound field reproduction which are arranged at four corners of a room. As result, many of imaginary sound sources distributed among the loudspeakers FL, FR, RL, RR are reproduced and excellent sound field reproduction appropriate to the selected music hall is brought about.
However, in such a conventional arrangement in the case where a sum signal (L+R) of individual channel signals for stereophonic signals is employed for the sound field reproduction, when the signal (L+R), which is a component localized in a center position of right and left, is inputted to the DSP 2, a component such as a conversation to be localized in the center position will also be scattered around and consequently may bring about unnatural sound field reproduction.
In contrast to this, where the difference signal (L-R) is employed, the component localized in the center position, such as a conversation, can secure the sound field reproduction as it is and does not cause the unnatural reproduction mentioned in the preceding paragraph. Nevertheless, since the energy of the component localized in the center position is not increased in intensity as compared with that of the component (L-R) other than the component localized in the center position, the component such as a conversation will relatively be weakened. Further, it may be more convenient to add appropriate sound field reproduction to the conversation per se. Although, for example, where a moving picture is viewed, it is effective to add the sound field coincident with a scene of the moving picture to its tone (corresponding to the component localized in the center position), the use of the signal (L-R) makes it impossible to meet such requirements. This is because only one of the signals (L+R) and (L-R) contributes to the sound field reproduction as the input of the DSP 2.